1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to a mobile communication system for switching communication channels between base stations covering respective communication areas as a mobile station moves between the communication areas.
2. Description of the Related Art
Mobile communication systems have a plurality of radio communication areas, each of a predetermined range, called cells, that cover an entire service area. Mobile stations in the service area can communicate with each other via base stations located in the respective cells. In such a mobile communication system, a channel switching control process is performed to switch base stations for radio communication with a mobile station as the mobile station moves from one cell to another. Such a channel switching control process is referred to as a hand-off control process. There are available two types of hand-off control processes, i.e., a hard hand-off control process with an instantaneous transmission break and a soft hand-off control process without an instantaneous transmission break.
The hard hand-off control process is simpler but causes an instantaneous transmission break because the present radio channel is cut off before the call is connected to a radio channel to switch to. The soft hand-off control process can switch radio channels without an instantaneous transmission break because the present radio channel remains connected when the call is connected to a radio channel to switch to.
For efficient usage of limited frequency resources, mobile communication systems allow mobile stations to make radio communication with base stations based on either the frequency division multiple access (FDMA) technique, the time division multiple access (TDMA) technique, or the code division multiple access (CDMA) technique. The CDMA mobile communication system often performs the soft hand-off control process free of an instantaneous transmission break using the same frequency in adjacent cells as the cells can easily share the same frequency band by spreading desired signals with respective inherent spreading codes.
FIG. 1 of the accompanying drawings schematically shows a conventional mobile communication system which performs a hand-off control process. As shown in FIG. 1, the conventional mobile communication system has first and second base stations 111, 112 covering respective first and second cells 101, 102 as their service areas. First and second base stations 111, 112 are connected to a mobile switching center 13 by respective communication channels 121, 122. Mobile switching center 13 is connected to public telephone switching network 15 by inter-switching-center line 14. First and second base stations 111, 112 that are located adjacent to each other are connected to each other by inter-base-station line 16. Mobile switching center 13 may include a base station controller for controlling a plurality of base stations. Communication channels 121, 122 and inter-base-station line 16 send and receive control information required for the hand-off control process. In the hand-off control process to be described below, it is assumed for illustrative purpose that mobile station 17 present in first cell 101 moves into second cell 102.
FIG. 2 of the accompanying drawings schematically shows a sequence of the hand-off control process carried out by the conventional mobile communication system shown in FIG. 1. Mobile station 17 is present in first cell 101, and is communicating with public telephone switching network 15 from first base station 111 via communication channel 121, mobile switching center 13, and inter-switching-center line 14 (communication 20). Mobile station 17 periodically measures a reception quality such as reception levels of radio waves received from surrounding base stations (measurement 21). If mobile station 17 detects, based on the measured results, that the channel quality between first base station 111 and mobile station 17 is lowered and the channel quality between second base station 112 and mobile station 17 is increased, then mobile station 17 sends quality information 22 representing such channel quality changes to first base station 111 which is master base station for mobile station 17.
When first base station 111 receives quality information 22, first base station 111 performs a soft hand-off control process, and transmits soft hand-off request 23 to second base station 112. Second base station 112 assigns an inherent spreading code and sets a radio channel between itself and mobile station 17. Mobile station 17 now enters a soft hand-off mode dominated by first base station 111 in which a radio channel is connected between mobile station 17 and first and second base stations 111, 112. Mobile station 17 then communicates with mobile switching center 13 via first and second base stations 111, 112 (communication 24).
In the soft hand-off mode dominated by first base station 111, since mobile switching center 13 recognizes that mobile station 17 is under the control of first base station 111, downstream user information is transmitted through a radio link from mobile switching center 13 via communication channel 121 to first base station 111 and then from first base station 111 to mobile station 17. At the same time, the downstream user information from mobile switching center 13 is transmitted via inter-base-station line 16 from first base station 111 to second base station 112, from which the downstream user information is also transmitted through a radio link to mobile station 17. Mobile station 17 receives the downstream user information from first base station 111 and second base station 112 by way of diversity reception. Mobile station 17 transmits upstream user information to first base station 111 and second base station 112. The upstream user information received by second base station 112 is transmitted via inter-base-station line 16 to first base station 111. First base station 111 receives the upstream user information by way of diversity reception, and transmits the received upstream user information via communication channel 121 to mobile switching center 13.
When mobile station 17 measures the channel quality (measurement 25) and detects that the channel quality between first base station 111 and mobile station 17 is further lowered and the channel quality between second base station 112 and mobile station 17 is sufficient, mobile station 17 sends quality information 26 representing such channel quality changes to first base station 111.
When first base station 111 receives the quality information, first base station 111 sends hard hand-off request 27 to mobile switching center 13 which is master station for first base station 111 in order to shift from the soft hand-off mode dominated by first base station 111 to a soft hand-off mode dominated by second base station 112.
In response to hard hand-off request 27 sent from first base station 111, mobile switching center 13 transmits hard hand-off instruction 28 via communication channels 121, 122 to first and second base stations 111, 112, thereby recognizing that mobile station 17 has been brought under the control of second base station 112. Mobile station 17 now enters the soft hand-off mode dominated by second base station 112 in which a radio channel is connected between mobile station 17 and first and second base stations 111, 112. Mobile station 17 then communicates with mobile switching center 13 via first and second base stations 111, 112 (communication 29).
When mobile station 17 measures the channel quality (measurement 30) and detects that the channel quality between first base station 111 and mobile station 17 is sufficiently lowered, mobile station 17 sends quality information 31 representing such channel quality changes to second base station 112.
Having received quality information 31, second base station 112 transmits a soft hand-off cancellation request 32 via inter-base-station line 16 to first base station 111 that is in the soft hand-off mode, finishes the soft hand-off mode, and enters a normal communication mode. Mobile station 17 can now communicate with public telephone switching network 15 via second base station 112, communication channel 122, mobile switching center 13, and inter-switching-center line 14 (communication 33).
In the above conventional mobile communication system, the inter-base-station line is provided between the adjacent base stations, and control information about the soft hand-off control of radio channels between the base stations and the mobile station and the hard hand-off control between the base stations and the mobile switching center is sent and received between the base stations. Therefore, the burden on the mobile switching center due to the soft hand-off control process can be reduced.
The technical details of the above mobile communication system are disclosed in Japanese unexamined patent publication No. 10-145834 on “Method of carrying out soft hand-off control in mobile communication system, mobile communication system and radio base station therefor”.
Published Japanese translation of PCT international publication No. 9-511107 on “Semi-hard hand-off in cellular electric communication system” discloses the technology of a mobile communication system for performing semi-hard hand-off control by placing a base station in an overlapping region covered by adjacent base stations under the control of different mobile switching centers, which serve as a master station for the base station placed in the overlapping region.
FIG. 3 of the accompanying drawings schematically shows an arrangement of the mobile communication system disclosed in the published Japanese translation of PCT international publication No. 9-511107. As shown in FIG. 3, the mobile communication system has first and second mobile switching centers 411, 412 connected to each other by switching center channel 40 and also connected to respective first and third base stations 431, 433 under its control by respective communication channels 421, 422. Second mobile switching center 412 is connected to public telephone switching network 45 via public network line 44. First and third base stations 431, 433 cover respective areas 461, 462, which have an overlapping region that is covered by second base station 432. Second base station 432 has, as its master station, first and second mobile switching centers 411, 412, and is connected thereto by respective communication channels 471, 472.
It is assumed that mobile station 48 has third base station 433 as its master station, is communicating with public telephone switching network 45 via second mobile switching center 412, and moves successively through the cells covered by second base station 432 and first base station 431. When mobile station 48 moves from the cell covered by third base station 433 to the cell covered by second base station 432, a soft hand-off control process is performed while mobile station 48 is being connected to second and third base stations 432, 433 via communication channels 422, 472. At this time, second mobile switching center 412 as a node receives information by way of diversity reception. As mobile station 48 moves away from third base station 433 toward second base station 432, the soft hand-off control process is finished, and mobile station 48 has second base station 432 as its master station and communicates with public telephone switching network 45 via second mobile switching center 412.
Mobile station 48 periodically reports measured reception quality to second base station 432 as its master station. If second mobile switching center 412 detects, based on the report, that mobile station 48 is moving from second base station 432 under its own control toward first base station 431 under the control of another mobile switching center, then second mobile switching center 412 starts a semi-hard hand-off control process. Specifically, second mobile switching center 412 detects that first base station 431 covering the cell toward which mobile station 48 is moving is under the control of first mobile switching center 411 adjacent to second mobile switching center 412, and sends a semi-hard hand-off request to first mobile switching center 411 via switching center channel 40. First mobile switching center 411, which is connected to second base station 432 by communication channel 471, transmits a semi-hard hand-off response to second mobile switching center 412 via switching center channel 40. In response to the semi-hard hand-off reply, second mobile switching center 412 disconnects communication channel 472 between itself and second base station 432. Mobile station 48 now has second base station 432 as its master station and communicates with public telephone switching network 45 via first mobile switching center 411, switching center channel 40, and second mobile switching center 412.
When mobile station 48 further moves toward first base station 431 under the control of first mobile switching center 411, a soft hand-off control process is carried out in which mobile station 48 is connected to first and second base stations 431, 432 via communication channels 421, 471, and first mobile switching center 411 as a node receives information by way of diversity reception.
In the above conventional mobile communication system, the hand-off control process performed when the mobile station moves is a soft hand-off control process without an instantaneous transmission break, rather than a hard hand-off control process with an instantaneous transmission break. Therefore, channels are provided between the base stations for performing the soft hand-off control process, between base station control apparatus, or between the mobile switching centers for sending and receiving user information. When the mobile station continues to move while in communication, the soft hand-off control process is repeated not only between the base stations, but also between the base station control apparatus as master stations for the base stations, and between the mobile switching centers as master stations therefor. When the mobile station further moves while the soft hand-off control process is being performed between the mobile switching centers, the mobile station may move from the service area of home system into the service area of another system.
A hand-off control process needs to be performed between different systems for increasing the quality of services provided to the users of mobile stations. In order to carry out such a hand-off control process between different systems, it is necessary to provide communication channels between mobile switching centers of one of the systems which control communications of the mobile station and adjacent mobile switching centers of the other system. However, attempts to provide communication channels between all adjacent mobile switching centers of the different systems would result in a high installation cost.